Orthogonal Frequency Division Multiplexing (“OFDM”) is a widely used signal multiplexing technique for communications, such as over-the-air (“wireless”) communications. An OFDM modulated signal is assembled at a baseband frequency associated with a communications channel as a weighted sum of N complex sinusoids, where N is an integer larger than or equal to one. Each complex sinusoid has a duration of T seconds with a frequency separation of 1/T Hz. The resulting OFDM configured signal spans a combined band of frequencies of N/T Hz.
An OFDM modulator assembles the OFDM modulated signal by spectral translation, summing real and imaginary components of the N complex sinusoids. Weighting terms for this weighted sum are independent random selections of complex data points in a so-called “constellation set” for modulations of the OFDM modulated signal. This OFDM constellation set may be modulated by Quadrature Amplitude Modulation (“QAM”).
Due to the additive summation of N sub-carriers to provide a weighted sum for QAM, relatively large peaks in the OFDM modulated signal may result, whether in a positive or negative direction. The average power of an OFDM modulated signal transmitted is maintained in a channel to obtain a threshold system error rate. The peak-to-average power ratio (“PAPR”) is conventionally used to define a dynamic range to be accommodated by components in a signal flow path down stream from the OFDM modulator. Conventionally, a PAPR is used though a peak-to-average ratio of some other energy level, such as voltage level for example, may be used. Clipping of peaks has been used to reduce the PAPR, which reduces the dynamic range, and thus also reduces the expense and complexity of components. Moreover, clipping of excessive peaks, which may cause interference in other carrier signals such those near the baseband frequency of the OFDM modulated signal for example, may reduce possible interference. However, clipping causes signal loss, and thus may negatively impact error rate performance.
Accordingly, it would be desirable to modify a time envelope of a time domain signal formed by random QAM of spectral terms of an OFDM modulated signal to reduce the PAPR. Such modification should not modify the spectral terms, as that may introduce additive interference which negatively impacts error rate performance.